As the semiconductor industry continues to increase circuit complexity and density by reduction of process node geometries, operating signal frequencies continue to increase. It is now possible to obtain semiconductors that operate well into the millimeter wave region of radio spectrum (30 GHz to 300 GHz). Traditionally the types of semiconductors used have been in the category of “III-V” types, indicating that the semiconductor compounds have been derived from period table elements in the third and fifth columns. Examples of these are gallium arsenide (GaAs) and indium phosphide (InP). In recent years, less expensive semiconductor processes that arise from column IV (such as silicon and germanium, Si and Ge) have been produced in silicon CMOS (complementary metal oxide semiconductor) and silicon germanium (SiGe) compounds. The result has been to extend the operating frequency of low-cost silicon well into the 60 to 80 GHz range of frequencies. By having low-cost semiconductor technology available, it has put pressure on the millimeter wave manufactures to bring the overall costs down for the electromechanical support mechanisms that enable these semiconductor devices.
Commercial waveguide structures enable low-loss energy transfer at millimeter wave frequencies, with the additional benefit of having been standardized on size and mechanical coupling flange designs. By having standardized sizes and coupling flanges, interoperability between different devices and different manufactures is enabled, providing maximal flexibility for millimeter wave system design. The traditional method for interfacing semiconductor devices within a mechanical waveguide has been to either provide a split-cavity type of assembly with expensive precision machining requirements or to couple energy from an orthogonal planar printed circuit launch probe with associated lossy energy transfer. In addition, with new semiconductor designs providing balanced transmission line outputs, there has been no straightforward electromechanical method for coupling millimeter wave energy from the balanced outputs directly to a waveguide without added circuitry such as a balun transformer which is also exhibits excessive losses as the frequency range of operation increases.
The prior art methods for coupling energy into and out of semiconductor devices can be divided into two categories. The first is the use of split-cavity metallic structures that allow the semiconductor chip to be placed into one of the cavities, with the other half of the cavity then brought together with the first half with precision fit. The typical precision required for the internal dimensions of a millimeter wave waveguide is on the order of ±0.001″ (0.025 mm). Holding this precision in the construction of the upper and lower cavity halves through machining, and maintaining registration alignment for assembly is expensive.
The second method used is to provide a printed circuit board with a stub or paddle energy launch. The stub or paddle launch is orthogonal to the waveguide cavity, also requiring a split-cavity type of assembly method.
In each case a custom, highly precision machining process is required to maintain the internal waveguide dimensional requirements. Some cost reduction can be afforded through a casting process, but secondary machining operations are still necessary to realize the precision needed.
The above methods are also designed for single-ended circuit configurations only. It is necessary to provide low-cost and efficient coupling methods for both single-ended and differential circuits. Millimeter wave semiconductor circuit designs often make use of differential amplifier and output stage configurations to enable high gain and power efficiencies.
What is needed is a low-cost and highly efficient coupling technique for semiconductor microwave, millimeter wave and sub-millimeter wave device energy transfer to and from standardized waveguide structures.